As one type of apparatuses (navigation apparatuses) for knowing the position of mobile object, such as vehicle, itself, and the speed thereof, there has been an inertial navigation system (INS) that does not require the help of ground facilities. An inertia sensor, which is employed for this INS, comprises an acceleration sensor for detecting the change of speed, and a gyroscope for detecting the change of attitude. The INS can know its own position and speed by accumulating the changes of speed and attitude from those of a departure point using an inertia sensor.
In order to specify the attitudinal state of mobile object, an orthogonally 3-axis coordinate system, in which the front/rear (axis) direction of mobile object is designated an x (roll) axis, the right/left (axis) direction is designated a y (pitch) axis and the up/down (axis) direction is designated a z (yaw) axis, has been used usually. When a mobile object is placed on a horizontal ground surface, it is assumed that the mobile object's yaw axis is directed to the vertical axis of the earth (Z axis) virtually, and that the roll axis and pitch axis become parallel to a local horizontal surface of the earth (X-Y plane) virtually. And, the attitude angle is expressed with three angles, a pitch angle, an angle that that the roll axis inclines from the local horizontal-surface X-axis, a roll angle, an angle that the pitch axis inclines from the local horizontal-surface Y-axis, and a yaw angle, an angle that the roll axis or pitch angle deviates on the X-Y plane. In order to measure these three angles, a 3-axis gyroscope has been used.
For example, in the invention of Patent Literature No. 1 an attitude angle is calculated by carrying out weighting to the output values of 3-axis gyroscope and acceleration sensor, respectively. The output results of the gyroscope are integrated for detecting an attitude angle. However, in gyroscopes, such a phenomena (drift) is present that they output errors in the same direction as the elapse of time, and accordingly the errors have become accumulated when integrating values that involve errors. In addition to above, since gyroscopes are expensive, a method is available in order to satisfy the functions at low cost, method in which gravity is detected with an acceleration sensor to calculate an attitude angle.
In the invention of Patent Literature No. 2, an attitude angle of mobile object is detected based on the output results of two acceleration sensors. However, since the outputs of the acceleration sensors involve accelerations being accompanied by the movement of the mobile object, it is difficult to detect the attitude angle accurately by the invention disclosed in Patent Literature No. 2.
Patent Literature No. 1: Japanese Unexamined Patent Publication (KOKAI) Gazette No. 8-178,687; and
Patent Literature No. 2: Japanese Unexamined Patent Publication (KOKAI) Gazette No. 2003-139,536